Molecular Beam Epitaxy (MBE) is a vacuum evaporation technique in which effusion cells containing elemental sources are used to generate molecular beam fluxes for ultra high vacuum, in situ, epitaxial growth of crystalline structures on uniformly heated substrates. Metals, semiconductors and insulators can be grown by MBE.
Of particular interest to researchers and developers, because of their excellent optoelectronic properties, is the epitaxial growth of III-V compounds and alloys thereof and, more specifically, GaAs and AlGaAs. Note that reference will be made hereafter to III-V materials in general but the invention will be described using the IIIA series of elements, i.e. Boron, Aluminum, Gallium, Indium and Thallium and, in particular, Gallium, as the III material; while the alloys of the III-V compound will refer principally to the aluminum alloy of GaAs or GaInAs.
Typical MBE systems include a high vacuum growth chamber in which a plurality of effusion cells are disposed. Each cell consists of a pyrolitic boron nitride crucible in which a charge of the element, and/or dopant to be deposited, is stored. Each crucible is heated by a furnace and has a shutter located in front of it to physically interrupt the beam flux. Adjacent cells are isolated from each other by cryopanels.
Instrumentation, such as reflection high-energy-electron diffraction (RHEED) equipment, is employed for in situ characterization of the structure during growth.
MBE routinely achieves abrupt heterointerfaces, precise thickness and doping control, which is essential for fabrication of a variety of semiconductor structures. Such structures include, for example, selectively doped heterostructure transistors (SDHT's) in which doping of heterostructures is modulated to spatially separate conduction electrons and their parent donor impurity atoms. Another example of MBE applications is the fabrication of superlattices or quantum-well structures consisting of alternate epitaxial layers of thin (5-40 nm) low band gap (GaAs) and high band gap (Al.sub.x Ga.sub.1(-x) As) materials. The abrupt steps in the energy gaps of the superlattices form potential wells in the conduction and valence bands. In the GaAs layers, the motion of the carriers is restricted in the direction perpendicular to the heterojunction interfaces, while they are free to move in the other two directions. The carriers (electrons, in this case) can therefore be considered as a two dimensional gas, sometimes referred to as a 2DEG.
Quantum well heterostructures exhibit more efficient luminescence intensities than bulk crystal heterostructures and have therefore been incorporated into the active region of laser devices.
Processes for growing quantum wells of III-V compound alloys with thicknesses and alloy content that vary across the substrate surface in a controlled manner, are not presently available. The ability to grow planar epitaxial layers of III-V alloys with regions of differing and controllable thicknesses and alloy content across a wafer would enable fabrication of a number of new or improved devices. These include the monolithic integration of lasers of different frequencies, high electron mobility transistors with tapered quantum wells, new waveguide devices, and improvement in spatial light modulators which use the excitonic Stark effect of quantum wells.
Tapered quantum wells in the GaAs/AlGaAs system have been achieved by growing upon substrates in which grooves have been etched [W. T. Tsang and A. Y. Cho, Appl. Phys. Lett. 30, 293 (1977 ); S. Nagata, T. Tanaka and M. Fukai, ti Appli. Phys. Lett. 30, 503 (1977); J. S. Smith, P. L. Derry, S. Margalit and A. Yariv, Appl. Phys. Lett. 47, 712 (1985); E. Kapon, M. C Tamargo and D. M. Hwang, Appl. Phys. Lett. 50, 347 (1987)], thereby relying on the growth rate differences between various crystallographic planes. This etched-groove process achieves tapering of layer thickness at the expense of planarity and creates thickness variations only at the edges and sidewalls of the grooves.
Other semiconductor growth processes, such as chemical vapor deposition (CVD); organo metallic CVD (OMCVD); and now recently chemical beam epitaxy (CBE), involve growth by deposition of III-V reactants on uniformly heated substrates. Such processes would also benefit from the ability to vary the thickness of the deposit and content of the III material in regions of the substrate. Accordingly, a long felt, heretofore unsatisfied, need has existed for a simple process of growing III-V compounds or alloys thereof on a substrate with regions of controlled thickness and alloy content.